Variable valve actuation mechanisms that change the valve actuation of engine valves such as intake valves and exhaust valves according to the engine operating state have been made commercially available.
Such variable valve actuation mechanisms include variable valve timing mechanisms. A variable valve timing mechanism uses hydraulic pressure generated by engine power or electricity as a drive source to change the rotational phase of a camshaft relative to the crankshaft, thereby changing the valve timing of the intake valves opened and closed by the camshaft to correspond to the operating state. Japanese Laid-Open Patent Publication No. 2001-263015 discloses a variable valve actuation mechanism that uses power source obtained from engine power to change the opening period and lift of engine valves to correspond to the engine operating state.
When a driver turns off the ignition switch, that is, when an engine stop request is made, fuel injection and fuel ignition are stopped so that the engine is stopped. Accordingly, a variable valve actuation mechanism is stopped while preserving the valve characteristics immediately before the engine stop. The valve characteristics after the engine is stopped are the same as the valve characteristics immediately before the engine is stopped, that is, the valve characteristics that have been set during the operation of the engine. These valve characteristics are not necessarily suitable for starting the engine. Thus, depending on conditions, startability of the engine could be degraded next time the engine is started.
Accordingly, Japanese Laid-Open Patent Publication No. 2002-161766 discloses an apparatus that performs a delay control for extending the period from when an engine stop request is made to when the engine is actually stopped. While the delay control is being performed, that is, while the power source from the engine power is available, a variable valve actuation mechanism is driven to change the valve characteristics to predetermined characteristics suitable for stating the engine.